Extern types don't codegen correctly when array-typed

[grebe]

Describe the bug Extern types (e.g. #[sv_type(...)]) don't codegen correctly for array types.

To Reproduce Given an sv type like typedef logic[7:0][3] u8_3_t;:

#[sv_type("u8_3_t")]
type u8_3_t = u8[3];

fn muladd(abc: u8_3_t) -> u8 {
    abc[0] * abc[1] + abc[2]
}

and run:

$BINPATH/xls/dslx/ir_convert/ir_converter_main test.x --top=muladd --interface_proto_file=test.proto | $BINPATH/xls/tools/codegen_main - --delay_model=asap7 --pipeline_stages=1 --ir_interface_proto=test.proto

Expected behavior Codegen should use abc[0], abc[1], abc[2], etc., but in reality you get

  wire [7:0] abc_unflattened[3];
  assign abc_unflattened[0] = abc[7:0];
  assign abc_unflattened[1] = abc[15:8];
  assign abc_unflattened[2] = abc[23:16];

Additional context The problem is that our codegen expects array ports to be packed into a bitvector and then internally unpacks the array, but using extern types violates this expectation.

A workaround is to wrap extern sv types in a struct.